Receiving part for connecting a shank of a bone anchoring element to a rod and bone anchoring device with such a receiving part

ABSTRACT

A receiving part for connecting a shank of a bone anchoring element to a rod. The receiving part has a first end, a second end and a recess having a substantially U-shaped cross section. The recess extends from the first end in a direction of the second end forming two legs open at the first end. The recess defines a channel to receive the rod. The channel has a longitudinal axis. The legs have an inner thread to receive a securing element having an outer thread cooperating with the inner thread. The legs are provided with stiffening structures. In one embodiment, the stiffening structures are formed by the wall thickness of the outer edges of the legs being larger than the wall thickness of the legs at a location closest to the central axis of the receiving part.

CROSS-REFERENCE TO RELATED APPLICATION(S)

This application is a continuation of U.S. patent application Ser. No.11/440,489, filed May 24, 2006, now U.S. Pat. No. 8,262,704, issued Sep.11, 2012, which claims the benefit of U.S. Provisional Application No.60/685,798 filed May 31, 2005 and claims the priority of European PatentApplication No. 05 011 438.8, filed May 27, 2005, the entire contents ofwhich are incorporated herein by reference.

BACKGROUND

The invention relates to a receiving part for connecting a shank of abone anchoring element to a rod and to a bone anchoring device with sucha receiving part to be used in spinal surgery or trauma surgery.

A known polyaxial bone screw 100 is shown in FIG. 11. It comprises areceiving part 101 for connecting a screw element 102 to a rod 103, theouter wall of the receiving part being substantially cylindrical. Thereceiving part 101 has a recess 104 with a substantially U-shapedcross-section forming two open legs 105, 106 defining a channel forinserting the rod 103. An inner thread 107 is provided at said legs forreceiving a securing screw 108 to fix said rod in the channel. The innerthread 107 is a metric thread.

When screwing in the securing screw 108 as shown in FIG. 13, forcesdirected radially outwards which are depicted by arrows A act on saidinner thread which cause said legs 105, 106 to splay. This may loosenthe fixation of the rod.

Several approaches have been made so far to solve the problem of thesplaying of the legs. One solution is to provide tube-shapedcounter-holding tools. Another solution consists in providing an outernut to be screwed onto an outer thread of said legs as, for example,disclosed in EP 0 614 649 A. Also, outer ring-shaped securing means areknown. However, securing means such as outer nuts or rings enlarge thesize of the bone screw and therefore limit the clinical applications.

A further approach consists in using a specific kind of thread shape forthe inner thread 107 and the corresponding thread of the securing screw.It is known to use a saw tooth thread as disclosed in U.S. Pat. No.5,005,562 or a reverse angle thread such as disclosed in U.S. Pat. No.6,296,642 B1 or US 2002/0138076 A1 to eliminate the outwardly directedradial forces acting on said legs. A particularly advantageous threadshape is the flat thread as disclosed in US 2003/0100896 A1 whicheliminates such radial forces and is easy to manufacture. The use ofthese specific thread shapes allows a use of the bone screw without afurther outer securing device such as an outer nut or ring.

However, despite the above measures there is still a problem of adiagonal splaying of the legs at the time of final tightening of thesecuring screw. This splaying is caused mainly by the torque acting onsaid legs 105, 106 at the time of final tightening which causes adiagonal lateral deformation of the open ends of the legs in thelongitudinal direction of the rod due to the friction between the threadflanks. The torque B acting on the legs is shown in FIG. 2. Thedeformation is shown in FIG. 14 which does not show the securing screwfor the purpose of better illustration. The problem of the splaying ofthe legs is not restricted to polyaxial bone screws but also occurs inall so called top open bone anchoring devices including monoaxial screwswhich have two open legs and a slit in between for inserting the rod.

Providing flattened surfaces 109 at the sides of the receiving partwhich are oriented in a transverse direction of the channel, as shown inFIG. 12, does not solve the problem but contributes to minimize the sizeof the bone screw in a longitudinal direction of the rod.

SUMMARY

It is an object of the invention to provide a receiving part, inparticular for a polyaxial bone anchoring device, and a bone anchoringdevice which is connected to a rod which is safe in use and at the sametime has a compact design. Further it is an object to provide a tool forholding and/or guiding such a bone anchoring device.

The receiving part according to the invention has the advantage that itprevents or significantly reduces splaying of the legs without using anyouter securing means. It is of a reduced size in a direction parallel tothe longitudinal axis of said rod. Therefore it is more compact than theknown screws. When designing it with the same size as a conventionalscrew in a direction parallel to the longitudinal axis of the rod, a rodwith a larger diameter compared to that of the conventional screw can beused.

Further features and advantages of the invention will become apparentfrom the description of the embodiments using the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a perspective view of a receiving part according to a firstembodiment of the invention.

FIG. 2 shows a top view of the receiving part according to the firstembodiment.

FIG. 3a shows a sectional view of the bone anchoring device with thereceiving part of the first embodiment.

FIG. 3b shows a perspective view of the bone anchoring device with thereceiving part of the first embodiment.

FIGS. 4a and 4b show schematically the principle of providing astiffening structure according to the first embodiment of the receivingpart.

FIG. 5a shows a schematic view of the cross section of the open legs ofa receiving part according to a second embodiment of the invention.

FIG. 5b shows a perspective view of the receiving part according to thesecond embodiment.

FIG. 6a shows a schematic view of the cross section of the open legs ofa receiving part according to a third embodiment of the invention.

FIG. 6b shows a perspective view of the receiving part according to thethird embodiment.

FIG. 7 shows a side view of a counter holding tool.

FIG. 8 shows a perspective view of a holding portion of a counterholding tool according to the invention.

FIG. 9 shows a perspective view of a conventional trocar tool.

FIG. 10 shows a perspective view of a trocar tool according to theinvention.

FIG. 11 shows a perspective view of a conventional polyaxial bone screw.

FIG. 12 shows a perspective view of a modified conventional polyaxialbone screw.

FIG. 13 shows a schematic sectional view illustrating a problem arisingwith the conventional receiving part.

FIG. 14 shows a schematic view illustrating a further problem arisingwith the conventional receiving part.

DETAILED DESCRIPTION

As shown in FIGS. 1 to 3 b a receiving part 1 for connecting a shank 50of a bone anchoring element 51 to a rod according to the inventioncomprises a first end 2 and a second end 3, a central axis M and arecess 4 having a substantially U-shaped cross-section. The recess 4extends from the first end 2 towards the second end 3 and forms two legs5, 6 open at the first end 2. The recess defines a channel having alongitudinal axis L for the insertion of a rod 7. The longitudinal axisL corresponds to the longitudinal axis of the rod when the rod isinserted. The width of the recess 4 is just as large that the rod can beplaced in and fit into the bottom 8 of the recess while still being ableto perform a sliding motion in the recess when it is not fixed. The legs5, 6 project above the rod when the rod is inserted and have an innerthread 9 extending from a position adjacent to the first end 2 in adirection of the second end 3 over a predetermined length to allow asecuring screw 10 to be screwed in to fix the rod. The inner thread 9and the matching outer thread of the securing screw is preferably a flatthread, the upper and lower flanks of which each include an angle of 90degrees with the screw axis.

At the outer wall surface of the legs 5, 6 two opposite flat surfaces11, 12 are provided which extend parallel to the longitudinal axis L ofthe channel and parallel to the central axis M of the receiving part.The flat surfaces 11, 12 are parallel to each other and extend from afirst position near the first end 2 to a second position towards thesecond end 3 which is located below the bottom 8 of the recess 4. Hence,the flat surfaces 11, 12 extend along the portions of said legs whichenclose the rod from both sides. The receiving part 1 further comprisesat its sides transverse to the longitudinal axis L of the recess twoopposite flat surfaces 13, 14 extending from a distance from the firstend 2 to a position below the bottom 8 of the recess. The flat surfaces13, 14 are parallel to the central axis M and to each other. In thisway, the flat surfaces 11, 12 and the flat surfaces 13, 14 include anangle of 90°, respectively. In the embodiment shown, the width W of theflat surfaces 11, 12, 13 and 14 in a direction perpendicular to thecentral axis M is the same so that an envelope E in a circumferentialdirection of the receiving part near the bottom 8 of the recess issubstantially square shaped.

At the lateral edges of the legs 5, 6 portions 15, 16, 17, 18 withenhanced wall thickness compared to the wall thickness of the legs inthe region of the flat surfaces 11, 12 are provided. These portions formstiffening structures preventing or significantly reducing a bending ofthe legs due to a diagonal splaying. The portions 15, 16, 17, 18 havingthe enhanced wall thickness have either a flattened outer surface asshown in FIGS. 1 and 2 or a rounded surface. They may extend along thewhole length of the flat surfaces 11, 12, 13 or 14 or, as shown in FIGS.1 and 2, go over in slanted portions 19 with diminishing wall thicknesstowards the first end 2.

FIGS. 3a and 3b show the receiving part according to the inventiontogether with a bone screw 51 to form a polyaxial bone anchoring device.In this case the receiving part 1 further comprises, as shown in FIG. 3a, a coaxial bore 20 extending through the bottom 8 of the channel andtapering in a section 21 towards the second end 3 to provide an opening22 in the second end 3 which has a diameter smaller than that of thebore 20. The diameter of said bore 20 is larger than the diameter of ahead 52 of the screw element 51 to allow the shank 50 and the screw head52 of the bone anchoring screw 51 to be guided through said bore. Thediameter of the opening 22 is smaller than the diameter of the screwhead 52 but larger than the diameter of the shank 50. Thus, the taperingsection 21 and the screw head 52 form a ball and socket joint toflexibly connect the bone anchoring screw and the receiving part.

A pressure element 25 is provided which is slidable in the bore 20 andserves for exerting pressure on the head to fix the screw element in anangular position with respect to the receiving part when the pressureelement is pressed against the head. The pressure element may besubstantially disc shaped and may have a recess for encompassing theupper part of the screw head and a recess for accommodating the lowerside of the rod 7. The pressure element may also have an opening forallowing a screwing-in tool to be guided through.

As can be seen from FIG. 3b the portion 15 of the receiving part andcorresponding portions 16, 17, 18, which have the enhanced wallthickness, are located approximately at the height of the longitudinalaxis of the rod when the rod is inserted into the receiving part and theflat surfaces 11, 12, 13, 14 extend over the area where the rod isenclosed by the legs 5, 6 from both sides.

In operation the screw element and the pressure element are insertedinto the receiving part. Then the rod is placed in and fixed by means ofthe securing screw. In the embodiment shown the rod presses on thepressure element when the inner screw is tightened and the screw headand the rod are fixed simultaneously.

As shown in FIG. 2 by means of the arrows B the torque acting on thelegs is more uniformly distributed so that a diagonal splaying of thelegs can be almost fully prevented.

FIGS. 4a and 4b schematically show the principle of enhancing thestiffness of the legs 5, 6 according to the invention. In theconventional receiving part shown in FIG. 4a the legs have across-section in a direction transverse to the central axis M shapedlike the shape of a section of a cylinder. Flattening the outer wall ofthe legs in a direction parallel to the longitudinal axis L of therecess 4 to generate the flat surfaces 11, 12 diminishes the wallthickness to a value d at a position closest to the central axis M. Thethickness d is smaller than the wall thickness in the region of thelateral edges of the legs. Further flattening the surfaces perpendicularto the longitudinal axis L of the recess 4 to create the flat surfaces13, 14 eliminates the sharp edges 30 and generates portions 15 withenhanced wall thickness D at the lateral edges of the legs. Thisprovides a stiffening structure to prevent diagonal splaying of thelegs. The following example illustrates the effect achieved with thereceiving part according to the invention. By using a rod with adiameter d_(rod) of 5.5 mm and with an inner diameter of 7 mm of theinner threaded portion, the lateral width W_(max) of the receiving partaccording to the invention in a direction transverse to the rod isreduced to 11.5 mm compared to 13 mm in the conventional receiving part.The geometrical moment of inertia which is a measure for the stiffnessof the flanks, is enhanced to 380 mm⁴ for the receiving part accordingto the invention compared to 240 mm⁴ in the conventional case.

The reduced lateral width W_(max) allows the construction of moreanatomically shaped lower sides of the bone anchoring devices whichcontributes to reducing the load on and/or the contact with the facetjoints.

On the contrary, if a receiving part according to the invention has thesame lateral width W_(max) as the conventional cylindrical receivingpart, the receiving part according to the invention can have a largerrecess 4 for the use of a rod with a larger diameter without decreasedstiffness.

The receiving part 1 may be produced by providing a cylindrical part andproducing the flat surfaces by milling.

The invention is not limited to the use of the receiving part in a boneanchoring device as shown in FIGS. 3a and 3b . Other polyaxialconnections are possible, for example the head and the rod may be fixedseparately. In this case separate rod fixation means and head fixationmeans are provided. Further, in the embodiment shown the screw head andthe tapering section are of a spherical shape, but any other shape ispossible as long as the head of the bone anchoring screw and thetapering section function as a ball and socket joint. For example, thetapering section can be conically shaped.

The invention is further not limited to polyaxial bone anchoring devicesbut is also applicable to monoaxial bone anchoring devices. In thiscase, the second end 3 of the receiving part is fixedly connected to ashank which is to be anchored in a bone.

The shank can be a threaded shank as shown in the Figures but may alsobe a pin or a hook or another device which is apt to be anchored in thebone.

The inner thread can have any shape, although a shape which reduces theforces directed radially outward is preferred.

A second embodiment of the receiving part is shown in FIGS. 5a and 5b .The second embodiment differs from the first embodiment shown in FIGS. 1to 3 b in that the surfaces 11′, 12′ corresponding to the flat surfaces11, 12 are not flat but concavely shaped and that the portions 15′having an enhanced wall thickness which are the outer edges of theconcave surfaces 11′, 12′ extend along the whole length of legs up tothe first end 2. Although stiffness is reduced compared to the firstembodiment, the splaying of the legs is still considerably reduced withthe receiving part of the second embodiment. The receiving part of thesecond embodiment has the advantage that the concave surfaces facilitategripping of the receiving part by the surgeon.

A third embodiment of the receiving part is shown in FIGS. 6a and 6b .The third embodiment differs from the first embodiment shown in FIGS. 1to 3 b in that the flat surfaces 11, 12 are provided with longitudinalribs 115 extending in a direction transverse to the longitudinal axis Lof the channel and parallel to the central axis M of the receiving partover the whole surface of the flat surfaces. The longitudinal ribsfacilitate gripping of the receiving part.

Other modifications are possible. The ribs do not need to extend overthe whole outer surface of the legs. Further, also the second embodimentmay have such longitudinal ribs on the concave surfaces 11′, 12′. It iseven conceivable to provide the ribs at the outer surfaces of theconventional, convexly shaped outer surfaces of the known receivingparts.

In a further modification the stiffening structures are realized byadding to the basic material from which the receiving part is formed oneor more additional materials to stiffen the basic material. This can bemade, for example, by adding fibers, for example carbon fibers, or byadding glass beads.

FIG. 7 shows a conventional counter-holding tool for use with receivingparts of polyaxial screws. The counter-holding tool has a grip portion30, a hollow shaft 32 which allows a screwing-in tool to be guidedthrough and a holding portion 33 which has a circular shape for use withthe conventional cylindrical receiving parts. The holding portion hasopposite slits 34 to fit over the rod which allow counter-holding of thereceiving part when the inner screw is screwed in.

According to the invention, the holding portion of the counter-holdingtool is adapted to the shape of the receiving part. FIG. 8 shows theholding portion 40 for a counter-holding tool according to theinvention. The holding portion 40 has a substantially square shapedcontour with a hollow interior with an opening 43 which is connected tothe hollow shaft of the counter-holding tool and two recesses 41 onopposite sides to fit onto the receiving part when the rod is inserted.The holding portion 40 further has two opposite parallel surfaces 42which cooperate with the flat surfaces 11, 12 of the receiving part toprovide a form-fit connection. The counter-holding tool is particularlysuitable for percutaneous applications in minimally invasive surgery. Bymeans of the form-fit connection between the holding portion and thereceiving part it is possible to precisely align the receiving parts.

FIG. 9 shows a conventional tube part of a trocar for minimally invasivesurgery which has a circular cross-section. FIG. 10 shows a tube part ofa trocar according to the invention which has a shape adapted to theshape of the receiving part which is substantially square. With the tubepart according to FIG. 10, due to the form-fit connection between thereceiving part and the tube part, it is possible, for example, toprecisely introduce and align the bone anchoring device by means ofpercutaneous introduction.

What is claimed is:
 1. A bone anchoring device comprising a boneanchoring element having a head and a shank, a rod, a receiving part forconnecting the bone anchoring element to the rod, and a securingelement, the receiving part comprising: a first end, a second end, and acentral axis extending between the first end and the second end, arecess having a substantially U-shaped cross section extendingtransversely to the central axis through the entire receiving part,wherein the recess is open at the first end of the receiving part andextends from the first end in a direction of the second end, wherein therecess forms two legs that come together to form a bottom of the recess,and wherein the recess defines a channel to receive the rod, and anouter surface that does not have a spherical profile extendingcontinuously from the first end of the receiving part to the second endof the receiving part, wherein the two legs are configured to receivethe securing element to secure the rod, and wherein the securing elementis rotatable to engage the two legs, wherein the receiving part ispolyaxially connectable to the bone anchoring element, wherein for atleast one vertical cross-sectional plane that includes the central axisand that extends through the legs, the resulting cross-section of thereceiving part has an outer profile with a first section, wherein anentire axial length of the first section has a maximum horizontal widthmeasured in a direction perpendicular to the central axis that isgreater than every other horizontal width of the outer profile, andwherein at least part of the first section is above the bottom of therecess, wherein the axial length of the first section is shorter than anaxial distance between the first end of the receiving part and the firstsection, and wherein the maximum horizontal width of the first sectionis greater than every other horizontal width of the entire portion ofthe receiving part between the first end of the receiving part and thefirst section.
 2. The bone anchoring device of claim 1, wherein the twolegs are provided with stiffening structures to prevent splaying causedby torque acting on the two legs, and wherein the stiffening structuresare formed at an outer wall of the two legs.
 3. The bone anchoringdevice of claim 2, wherein the stiffening structures comprise oppositeflat surfaces, each of the surfaces formed in a respective outer wall ofa leg of the two legs, the surfaces being parallel to a longitudinalaxis of the channel.
 4. The bone anchoring device of claim 3, whereinthe flat surfaces extend from above a bottom of the recess to below thebottom of the recess.
 5. The bone anchoring device of claim 4, whereinthe flat surfaces extend substantially up to the first end.
 6. The boneanchoring device of claim 3, wherein opposite flat surfaces are providedon sides of the receiving part perpendicular to the longitudinal axis ofthe channel.
 7. The bone anchoring device of claim 1, wherein oppositeflat surfaces are provided on sides of the receiving part perpendicularto a longitudinal axis of the channel.
 8. The bone anchoring device ofclaim 1, wherein at least one envelope of the receiving part issubstantially square shaped.
 9. The bone anchoring device of claim 1,wherein the two legs are provided with stiffening structures to preventsplaying caused by torque acting on the two legs, and wherein thestiffening structures are rib-like sections of enhanced wall thickness.10. The bone anchoring device of claim 9, wherein the rib-like sectionsextend substantially in a direction of the first end to the second end.11. The bone anchoring device of claim 10, wherein the stiffeningstructures are provided at a location so as to enclose the rod from bothsides.
 12. The bone anchoring device of claim 9, wherein the stiffeningstructures are provided at a location so as to enclose the rod from bothsides.
 13. The bone anchoring device of claim 1, wherein lateral edgesof the legs comprise a section of enhanced material thickness.
 14. Thebone anchoring device of claim 1, the receiving part further having abore extending from the first end to the second end for passing throughthe shank of the bone anchoring element.
 15. The bone anchoring deviceof claim 1, wherein the two legs are provided with stiffening structuresto prevent splaying caused by torque acting on the two legs, and whereinthe stiffening structures are formed by the wall thickness of outeredges of the legs being larger than a wall thickness of the legs at alocation between the outer edges of the legs, the outer edges spacedfrom each other in a direction of a longitudinal axis of the channel.16. The bone anchoring device of claim 1, wherein the two legs areprovided with stiffening structures to prevent splaying caused by torqueacting on the two legs, and wherein the stiffening structures compriseadditional material to stiffen a base material of the legs.
 17. The boneanchoring device of claim 16, wherein the additional material is locatedonly in a portion of the legs.
 18. The bone anchoring device of claim 1,wherein when at least a portion of the securing element is in thereceiving part and an axial force is exerted on the securing element ina direction away from the second end of the receiving part, the securingelement does not apply a force on the legs to urge the legs closer tothe central axis.
 19. A bone anchoring device comprising a boneanchoring element having a head and a shank, a rod, a receiving part forconnecting the bone anchoring element to the rod, and a securingelement, the receiving part comprising: a first end, a second end, acentral axis extending between the first end and the second end, and arecess having a substantially U-shaped cross section extendingtransversely to the central axis through the entire receiving part todefine a channel for receiving the rod, wherein the recess is open atthe first end of the receiving part and extends from the first end in adirection of the second end, and wherein the recess forms two legs thatcome together to form a bottom of the recess, wherein the securingelement is rotatable relative to the receiving part to engage respectiveengagement portions on the two legs for securing the rod and the head inthe receiving part, wherein the receiving part is polyaxiallyconnectable to the bone anchoring element, wherein along a first planethat is perpendicular to the central axis, that is spaced apart axiallyfrom the engagement portions, and that is positioned between theengagement portions and the bottom of the recess, each of the legs has afirst thickness in a first radial direction from the central axis and asecond thickness that is less than the first thickness in a secondradial direction from the central axis, and wherein along a second planethat is parallel to the first plane, that is spaced apart axially fromthe engagement portions, and that is positioned between the engagementportions and the first plane, respective thicknesses of each of the legsin every radial direction are less than the first thickness, while eachof the legs maintains the second thickness in the second radialdirection.
 20. The bone anchoring device of claim 19, wherein the twolegs are provided with stiffening structures to prevent splaying causedby torque acting on the two legs, and wherein the stiffening structurescomprise opposite flat surfaces, each of the surfaces formed in arespective outer wall of a leg of the two legs, the surfaces beingparallel to a longitudinal axis of the channel.
 21. The bone anchoringdevice of claim 20, wherein the flat surfaces extend from above a bottomof the recess to below the bottom of the recess.
 22. The bone anchoringdevice of claim 20, wherein opposite flat surfaces are provided on sidesof the receiving part perpendicular to the longitudinal axis of thechannel.
 23. The bone anchoring device of claim 19, wherein oppositeflat surfaces are provided on sides of the receiving part perpendicularto a longitudinal axis of the channel.
 24. A bone anchoring kit,comprising a bone anchoring element having a head and a shank, a rod, areceiving part for connecting the bone anchoring element to the rod, asecuring element, and a tool having a holding portion with a shapeconfigured to receive the receiving part, the receiving part comprising:a first end, a second end, and a central axis extending between thefirst end and the second end, a recess having a substantially U-shapedcross section extending transversely to the central axis through theentire receiving part, wherein the recess is open at the first end ofthe receiving part and extends from the first end in a direction of thesecond end, wherein the recess forms two legs that come together to forma bottom of the recess, and wherein the recess defines a channel toreceive the rod, and an outer surface that does not have a sphericalprofile extending continuously from the first end of the receiving partto the second end of the receiving part, wherein the two legs areconfigured to receive the securing element to secure the rod, andwherein the securing element is rotatable to engage the two legs,wherein the receiving part is polyaxially connectable to the boneanchoring element, wherein for at least one vertical cross-sectionalplane that includes the central axis and that extends through the legs,the resulting cross-section of the receiving part has an outer profilewith a first section, wherein an entire axial length of the firstsection has a maximum horizontal width measured in a directionperpendicular to the central axis that is greater than every otherhorizontal width of the outer profile, and wherein at least part of thefirst section is above the bottom of the recess, and wherein the axiallength of the first section is shorter than an axial distance betweenthe first end of the receiving part and the first section, and whereinthe maximum horizontal width of the first section is greater than everyother horizontal width of the entire portion of the receiving partbetween the first end of the receiving part and the first section.
 25. Abone anchoring device comprising a bone anchoring element having a headand a shank, a rod, a receiving part for connecting the bone anchoringelement to the rod, and a securing element, the receiving partcomprising: a first end, a second end, and a central axis extendingbetween the first end and the second end, a recess having asubstantially U-shaped cross section, wherein the recess is open at thefirst end of the receiving part and extends from the first end in adirection of the second end, wherein the recess forms two legs that cometogether to form a bottom of the recess, and wherein the recess definesa channel to receive the rod, and wherein the two legs are configured toreceive the securing element to secure the rod, and wherein the securingelement is rotatable to engage the two legs, wherein the receiving partis polyaxially connectable to the bone anchoring element, and whereinfor at least one vertical cross-sectional plane parallel to the centralaxis and that extends through the legs, the resulting cross-section ofthe receiving part has an outer profile with a portion that expands inwidth along a direction parallel to the central axis, with a rate ofexpansion that decreases and then increases as the cross-section extendsfrom the second end of the receiving part towards the first end of thereceiving part.
 26. A bone anchoring device comprising a bone anchoringelement having a head and a shank, a rod, a receiving part forconnecting the bone anchoring element to the rod, and a securingelement, the receiving part comprising: a first end, a second end, and acentral axis extending between the first end and the second end, arecess having a substantially U-shaped cross section, wherein the recessis open at the first end of the receiving part and extends from thefirst end in a direction of the second end, wherein the recess forms twolegs that come together to form a bottom of the recess, and wherein therecess defines a channel to receive the rod, the channel having alongitudinal axis, and wherein the two legs are configured to receivethe securing element to secure the rod and the securing element isrotatable to engage the two legs, and wherein when at least a portion ofthe securing element is in the receiving part and an axial force isexerted on the securing element in a direction away from the second endof the receiving part, the securing element does not apply a force onthe legs to urge the legs closer to the central axis, wherein thereceiving part is polyaxially connectable to the bone anchoring element,and wherein the receiving part has a horizontal first width that is amaximum outer width of the entire portion of the receiving part that isbetween the bottom of the recess and the second end, wherein for avertical cross-sectional plane that includes the central axis and thatis perpendicular to the longitudinal axis, a maximum outer width of thereceiving part is less than the first width, while a maximum outer widthof the receiving part in at least one other vertical cross-sectionalplane that includes the central axis is greater than the first width.27. A bone anchoring device comprising a bone anchoring element having ashank, a receiving part for connecting the shank of the bone anchoringelement to a rod, and a securing element having an outer thread, thereceiving part comprising: a first end and a second end and a centralaxis extending from the first end in a direction of the second end, arecess having a substantially U-shaped cross section, the recessextending from the first end in the direction of the second end andforming two legs open at the first end and coming together at a bottomof the recess, the recess defining a channel to receive the rod, thechannel having a longitudinal axis, and an outer surface that does nothave a spherical profile extending continuously from the first end ofthe receiving part to the second end of the receiving part, the two legshaving an inner thread to receive the securing element, the outer threadconfigured to cooperate with the inner thread to secure the rod, whereinthe receiving part is polyaxially connectable to the bone anchoringelement and the two legs are provided with stiffening structures toreduce splaying caused by a torque acting on the two legs at the time offinal tightening of the rod by the securing element, such that along atleast one vertical plane perpendicular to the longitudinal axis of thechannel, a cross-section of the receiving part includes a first portionabove the bottom of the recess and below the inner thread having agreatest outer width of the cross-section in a direction perpendicularto the central axis that is greater than respective outer widths ofevery portion of the cross-section below the bottom of the recess. 28.The bone anchoring device of claim 27, wherein the entire first portionhaving the greatest outer width is spaced apart axially from the innerthread.
 29. The bone anchoring device of claim 27, wherein the width ofthe first portion is greater than respective outer widths of everyportion of the cross-section above a bottom of the inner thread.
 30. Thebone anchoring device of claim 27, wherein the outer width of thecross-section of the receiving part becomes smaller from the firstportion in a direction towards the first end of the receiving part. 31.A bone anchoring device comprising a bone anchoring element having ashank, a receiving part for connecting the shank of the bone anchoringelement to a rod, and a securing element having an outer thread, thereceiving part comprising: a first end and a second end and a centralaxis extending from the first end in a direction of the second end, arecess having a substantially U-shaped cross section, the recessextending from the first end in the direction of the second end andforming two legs open at the first end and coming together at a bottomof the recess, the recess defining a channel to receive the rod, thechannel having a longitudinal axis, and an outer surface that does nothave a spherical profile extending continuously from the first end ofthe receiving part to the second end of the receiving part, the two legshaving an inner thread to receive the securing element, the outer threadconfigured to cooperate with the inner thread to secure the rod, whereinthe receiving part is polyaxially connectable to the bone anchoringelement and the two legs are provided with stiffening structures toreduce splaying caused by a torque acting on the two legs at the time offinal tightening of the rod by the securing element, such that along atleast one vertical plane perpendicular to the longitudinal axis of thechannel, a cross-section of the receiving part includes a portion abovethe bottom of the recess and below the inner thread where each of thelegs has a greatest wall thickness in a direction perpendicular to thecentral axis that is greater than respective wall thicknesses of everyportion of the cross-section below the bottom of the recess and of everyportion of the cross-section above a bottom of the inner thread.
 32. Thebone anchoring device of claim 31, wherein the bone anchoring elementfurther comprises a spherical head that is insertable into the receivingpart from the second end of the receiving part.
 33. The bone anchoringdevice of claim 32, wherein the head and the shank of the bone anchoringelement are monolithic.
 34. The bone anchoring device of claim 31,further comprising a pressure element that is movable axially in thereceiving part when the bone anchoring element and the receiving partare connected to one another, wherein the pressure element is configuredto exert pressure on the bone anchoring element to lock a position ofthe bone anchoring element relative to the receiving part.